1. Field of the Invention
This invention relates to the conversion of methane to C4+ products in high yields, and, in particular, to methods and systems for the steady state conversion of methane to C4+ aliphatic products in high yields using an integrated recycle reactor.
2. Description of the Background
Several recent studies have demonstrated that product yields approaching 70% may be achieved during the oxidative coupling of methane (OCM) by employing a recycle reactor with continuous removal of ethylene (Y. Jiang et al., Science 264:1563, 1994; R. B. Hall et al., ACS Div. Petr. Chem. Prepr. 39(2):214, 1994; E. M. Cordi et al., Appl. Catal. A: Gen. 155:L1-L7, 1997; A. Mashocki, Appl. Catal. A: General 146:391, 1996). The ethylene may either be directly separated from the recycle stream or it may be converted to another product, which is subsequently separated. As an example of the former, Ag+ ions have been used to facilitate the transport of ethylene and a small amount of propylene through a membrane contactor. The olefins were recovered in nearly pure form by heating the aqueous silver-olefin complexes to 100xc2x0 C. As an example of the second alternative, the ethylene was converted to aromatics (mainly benzene and toluene) over a Ga/H-ZSM-5 zeolite. The aromatics were separated from the recycle stream by cryogenic means.
However, in both of these cases, chemicals rather than fuels were produced. With respect to the utilization of methane in large remote gas fields, it is desirable to form a liquid transportation fuel that has a minimal amount of aromatics.
A two-step process for converting methane to liquid hydrocarbons in the gasoline range using oxidative coupling over a catalyst, followed by passing the product stream over a solid acid catalyst has been described (U.S. Pat. No. 5,336,825 to Choudhary et al.). However, this process does not recycle unreacted methane and ethane and produces products in low yields.
There is therefore a need for systems and methods for converting methane in high yields to C4+ nonaromatic hydrocarbons.
The present invention overcomes the problems and disadvantages associated with current strategies and designs and provides systems and processes for converting methane in high yields to C4+ nonaromatic hydrocarbons, producing high yields of hydrocarbons in the C4+ range.
Accordingly, one embodiment of the invention is directed to a process or method for producing C4+ aliphatic hydrocarbons in high yields from methane comprising the steps of: providing a stream of methane and a stream of O2; conveying the stream of methane and stream of O2 to an oxidative coupling reactor (OCM reactor), for the oxidative coupling of methane; converting the methane to ethylene and other by-products in the oxidative coupling reactor, thereby forming an ethylene-containing product stream; conveying the ethylene-containing product stream to a catalytic reactor; oligomerizing the ethylene in the catalytic reactor by reacting the ethylene over an acidic pentasil zeolite catalyst to produce an end product-containing stream, the end product-containing stream comprising hydrocarbons having four or more carbons; removing the hydrocarbons having four or more carbons from the end product-containing stream leaving an effluent stream; and recycling the effluent stream through the oxidative coupling reactor.
Another embodiment of the invention is directed to a process or method for producing C4+ hydrocarbons in high yields from methane comprising the steps of: reacting methane and O2 in an oxidative coupling reactor to produce a first product stream, the first product stream comprising ethylene; reacting the first product stream in a catalytic reactor in the presence of an acidic pentasil zeolite catalyst to produce a second product stream, the second product stream comprising C4+ hydrocarbons; removing C4+ hydrocarbons from the second product stream leaving an effluent stream; and recycling the effluent stream through the oxidative coupling reactor.
Another embodiment is directed to a system for producing C4+ hydrocarbons in high yields from methane comprising: an oxidative coupling reactor, the oxidative coupling reactor adapted to receive methane and O2 and to convert the methane to a first product stream, the first product stream containing ethylene; a catalytic reactor containing an acidic pentasil zeolite catalyst disposed downstream from the oxidative coupling reactor for receiving the first product stream and converting it to a second product stream, the second product stream containing C4+ hydrocarbons; means for removing C4+ hydrocarbons from the second product stream to produce an effluent stream; and means for recycling the effluent stream to the oxidative coupling reactor. Preferably, the apparatus further comprises means for removing water from the first product stream, means for removing CO2 from the first product stream, and means for removing by-products from the first product stream.
Other embodiments and advantages of the invention are set forth in part in the description which follows, and in part, will be obvious from this description, or may be learned from the practice of the invention.